This invention relates generally to non-impact printers and more specifically to an electrophotographic printer adapted to print copy from a source of image information.
In electrophotographic printing, a photoreceptor in the form of a photoconductive surface on a moving belt or drum is uniformly charged and exposed to a light image from a source of image information. Exposure of the photocoductive surface to a light source discharges localized portions of the surface and thereby records thereon an electrostatic latent image corresponding to the original image. The electrostatic latent image is then developed and rendered visible by depositing toner particles which adhere electrostatically thereto in an image configuration. Subsequently, the toner powder is transferred to a sheet of support material which may be plain paper or transparent plastic sheet, among other materials. The toner powder is then affixed to the support material and fused thereto so as to produce a so-called hard copy of the original image.
As a source of illumination for exposing the photoconductive surface in medium to high speed applications requiring high image quality, it is desirable to employ an array of light emitting diodes (LEDs). This permits the construction of a printer which is compact, light in weight, economical and having low maintenance while maintaining high quality printing. Such a non-impact printer employing light emitting diodes as a source of illumination can be used as peripheral equipment for the output of a computer where the array of light emitting diodes is driven by a digital signal from a character generator containing image or character information received from the computer. The source of image information for the light emitting diode array can also be from a hard copy reader such as an optical character reader, or from a high speed facsimile scanner, word processor or other image information generating device.
In such printers, the photoreceptor may be either a belt or a drum having a photoconductive surface thereon in optical registration with the light emitting diode array. Selective energization of individual groups of the light emitting diode array illuminates and discharges selected portions of the charged photoconductive surface in the image pattern of a line of print characters or other image associated with the image code signals output from the character generator or other source of image information. The selective energization of individual groups of the light emitting diode array produces an array of dot sized discharged portions of the photoconductive surface of the photoreceptor which has been charged prior to exposure to the diode array. The image created on the photoreceptor is in the form of depleted charge portions on the previously charged photoreceptor. As noted above, the latent image on the photoreceptor is developed electrophotographically by means well known in the art and not forming part of this invention.
It has been determined that a major cause of poor image quality in the printing of hard copy by such photoprinters is irregular motion by the photoreceptor. The photoreceptor is driven by an electrical or an electromechanical drive and as with any such mechanisms develops occasional jitter or other non-uniform irregular motion which, though slight, is capable of distorting or diminishing image quality of the output from the printer. It has been discovered that such poor image quality can be overcome by coordinating or synchronizing the energization of individual groups of the diode array with the motion of the photoreceptor even though the photoreceptor motion be non-uniform. In this way the dots forming the image on the photoreceptor will always be in the correct position to insure a high quality image despite irregular or non-uniform motion of the photoreceptor.
Accordingly, it is an object of this invention to provide a method and apparatus for synchronizing the energization of individual groups of the light emitting diode array with the motion of the photoreceptor to coordinate the exposure of selected localized areas of the photoconductive surface to produce a high quality latent image even though movement of the photoreceptor is irregularly non-uniform.